Beam riding missiles are guided to a target by following a beam pattern emitted from the launch pad and aiming at the target or at an estimated position of the target in the future. “Line of Sight” is the method by which a missile is guided to current position of the target. “Line of Command” is the method by which a missile is guided to an estimated position of the target in the future. The beam pattern can be made of as many beams as necessary. Often, data is encoded in some of the beams to communicate with airborne systems onboard the missile. Such a method involves the use of facilities to detect and track the target.
Monopulse clusters are used to detect and track the target and guide the missile using the same facilities alternately. Monopulse cluster is a well-known means to emit single frequency electromagnetic beams in different directions. It is based on waveguide technology. Range of frequency depends on the size of the waveguides making up the monopulse cluster. Directions of emission depend on the construction of the emission horn in combination with the parabola reflector. In a single antenna it is possible to alternately detect target and guide missile at a relatively low cost. The square-shaped beam position pattern resulting from the squared structure of 4 waveguides merged within an emission horn is used to guide missiles, assuming the missile is illuminated by at least some beams anywhere within the square.
Unfortunately, due to physical limitations, ends of waveguides merged within the horn of a cluster seldom generate beams that overlap correctly. As a consequence, a shadow zone in which the missile receives no beam at all may exist and in which estimation of a relative position of the missile is not possible. The shadow zone is located in the vicinity of the centre of the square, where deviation is minimum. The worst is that missile may miss control data encoded in one of the beams, preventing airborne control system onboard the missile from full operation.
Indeed, it is rather difficult to manufacture a four-output horn generating small angular distance between the four beams. The 4 beams generated are more separated than required for guidance. Using dielectric material instead of air within the waveguides, the spacing between the waveguides can be reduced. But resulting clusters are far more expensive and antennas are difficult to tune.